Indeed, less rigidity and higher tolerances lead to reliability - similar to what we do in construction of buildings: a skyscraper would fall one day if it wasn't for its flexibility under effects of elements such as wind.
That's not an apt analogy. A system with tight tolerances can still be flexible, we just know more precisely how it can flex and when it will break.
A better analogy would be if your construction workers didn't have standard or prescribed bolts in their design, so they just take what's lying around and hammer and weld bits together until it seemed sturdy enough. Suffice it to say, this is not a recipe that would work to build today's sky scrapers. There is considerable design and sanity checking that goes into this stuff which the web at every point completely lacked.
XHTML was a promising start in the right direction, but they unfortunately bungled it.
Interesting related trivia: engineers build safeguards around that flexibility - in the same way that a poorly built bridge will shake itself apart in the wind, a building without adaptive dampening or the right properties of flexibility could shake itself apart in the wind.
